Semiconductor memories such as MRAM (magnetic random access memory) using a resistance change element as a memory element have been developed recently. An MRAM is a memory storing data “1” or data “0” in memory cells by using magnetoresistive effects, and has nonvolatility, high-speed operation, high integration, and high reliability. Therefore, the MRAM is a potential candidate for a memory device substituting an SRAM, a PSRAM (Pseudo SRAM), a DRAM and the like.
There have been many reports of an MRAM using an element demonstrating a TMR (tunneling magnetoresistive) effect among the magnetoresistive effects. An MTJ (magnetic tunnel junction) element using the TMR effect has a laminate structure including two ferromagnetic layers and a nonmagnetic layer (an insulating thin film) sandwiched between these ferromagnetic layers, and stores digital data based on a change of a magnetoresistance due to a spin-polarized tunneling effect. The MTJ element can take a low resistance state and a high resistance state by a magnetization arrangement of the two ferromagnetic layers. One-bit data can be recorded in the MTJ element, by defining a low resistance state as data “0” and defining a high resistance state as data “1”.
In recent years, downscaling of MRAMs has been progressed to increase the data memory capacity, to reduce power consumption, and to reduce the manufacturing cost. As one of means for downscaling the MRAM, there is a configuration of arranging an MTJ element on a contact plug between adjacent two switching transistors. In this case, the roughness of an upper surface of the contact plug affects an electric property and a magnetic property.
Generally, an aspect ratio of a trench between switching transistors is formed large to downscale an element and to reduce resistances of word lines. Therefore, both or either one of a void and a seam occurs in a contact plug when a conductive material (tungsten, for example) as a material of the contact plug is embedded between adjacent switching transistors. Both or either one of the void and the seam becomes a cause of a dip on the upper surface of the contact plug, and causes a negative influence on the flatness of the upper surface of the contact plug.
When the flatness of the upper surface of the contact plug is poor, an uneven shape of the upper surface of the contact plug is transferred to the MTJ element on the contact plug. That is, the flatness of the insulating thin film sandwiched between the two ferromagnetic layers of the MTJ element also becomes poor. In this case, when a voltage is applied to the MTJ element, electric fields are locally concentrated in the insulating thin film, and this has a possibility of destructing the insulating thin film. Consequently, the roughness of the upper surface of the contact plug causes a negative influence on the electric property and the magnetic property of the MTJ element, and affects the reliability of the MRAM.
When there is a large dip on the upper surface of the contact plug, the MTJ element on the contact plug is inclined to a substrate surface in some cases due to misalignment of lithography or the like. When the MTJ element is inclined, a protection film or the like covering the MTJ element gives a bias stress to the MTJ element, and this has a possibility of degrading the electric property or the magnetic property of the MTJ element. Consequently, the inclination of the MTJ element also causes a negative influence on the reliability of the MRAM.